Settling down of seasonal migrants promotes bird diversification
How seasonal migration originated in birds, and how it impacted diversification, remain largely unknown. Although migratory behaviour is likely to affect bird diversification in various ways, previous studies have not detected any significant effect. For example migration can potentially enhance speciation, because different populations within the same species can have different migratory flyways or strategies, which can lead to genetic divergence between populations. Conversely, migration could reduce opportunities for speciation as it may increase gene flow between breeding populations, thereby reducing genetic divergence between populations. Seasonal migration can potentially also have effects on extinction. During past glaciation cycles in particular, migratory species were more likely than sedentary ones to escape from changing environmental conditions and to avoid extinction. Here, we infer ancestral migratory behaviour and the effect of seasonal migration on speciation and extinction using an almost complete bird tree of life.
Our analyses infer that the sedentary behaviour is ancestral, and that the migratory behavior originated repeatedly and independently during birds evolution. The speciation of a sedentary species into two sedentary daughter species is more frequent than that of a migratory species into two migratory daughter species. However, migratory species often diversify by generating a sedentary daughter species in addition to the ancestral migratory one. This leads to an overall higher migratory speciation rate. Migratory species also experience lower extinction rates. Hence, although migratory species represent a minority of all extant birds, they have a higher net diversification rate than sedentary species. These results suggest that the evolution of seasonal migration in birds has facilitated diversification through the divergence of migratory subpopulations that become sedentary, and illustrate asymmetrical diversification as a potentially under-recognized mechanism by which diversification rates are decoupled from species richness.